Project Description/Summary Gastrointestinal stromal tumor (GIST) is a rare type of cancer that affects approximately 40,000 patients with an annual incidence of 5,000 cases in the US. It arises from the ?pacemaker? cells of the gastrointestinal tract and is mainly characterized by activating mutations in KIT or PDGFRA receptor tyrosine kinases. Despite the initial clinical success of imatinib that targets mutant KIT/PDGFRA, nearly all advanced GIST patients develop imatinib-resistance and eventually die of their disease. It is critical to gain a better understanding of the pathogenesis of GIST and to develop novel treatment strategies that are 1) more effective than first-line imatinib therapy and 2) can delay and/or prevent imatinib-resistance. ETV1, an ETS family transcription factor and a well-established oncogene in prostate cancer and melanoma, has recently been discovered to play a critical role in GIST oncogenesis. ETV1 is highly expressed and is required for growth and survival of GISTs. ETV1 is a master regulator of the ICC-GIST lineage and is required for GIST tumor initiation and maintenance in vivo. ETV1 and mutant KIT form a positive feedback circuit in GIST oncogenesis, where the ETV1 protein is stabilized by active MAP kinase signaling downstream of KIT signaling and stabilized ETV1 in turn upregulates KIT expression. Hence, ETV1 represents a novel drug target. We demonstrated that the combination of MEK162 and imatinib can durably inhibit ETV1 protein and lead to enhanced apoptosis in GIST cells and complete responses of GIST tumors. These preclinical data led to an investigator initiated ?phase Ib/II study of MEK162 in combination with imatinib in patients with untreated advanced GIST? to directly evaluate the safety and clinical efficacy of this novel combination therapy in advanced GIST. The phase Ib portion of the study has been completed and has demonstrated the safety and defined the recommended phase II doses of the combination therapy in GIST patients. The phase II study is currently accruing and forms the basis of this proposal. The primary goal of the phase II study is to evaluate the efficacy of the combination of MEK162 and imatinib by RECIST responses in untreated advanced GIST patients. The phase II trial included secondary endpoints that evaluate the progression free survival, overall survival and respectability rate. There are mandatory pre- and post-treatment biopsies and biopsies at disease progression for correlative studies that explore the effect of the combination therapy in ETV1 target inhibition: a) inhibition of the ETV1 protein level, and 2) inhibition of the ETV1-dependent transcriptome. Additionally, we will examine the genetic tumor heterogeneity and genetic basis of resistance mechanisms to the combination therapy from the plasma tumor-derived cell-free DNA and clinical samples obtained at disease progression. We believe that targeting ETV1 by the combination treatment strategy represents a novel approach in GIST therapeutics. The phase II clinical trial, if successful, has the potential to revolutionize the first line therapy of GIST treatment and change the landscape of clinical practice in GIST management.